The present invention relates to fine barium titanate particles, and more particularly, to fine barium titanate particles which are free from aggregation therebetween and exhibit an excellent dispersibility.
With the recent tendency of miniaturization, high-performance and lightening of various electronic equipments, it has been required to improve properties of barium titanate particles used as a dielectric material for electronic parts, for example, laminated ceramic capacitors, etc.
As known in the art, in the conventional laminated capacitors, there have been frequently used barium titanate particles which are in the form of a perovskite compound having a high dielectric constant. The barium titanate particles having a tetragonal system are used as a ferroelectric substance, and have been applied to electronic parts such as ceramic capacitors, positive thermistors and high-frequency oscillators.
The barium titanate particles used for the laminated capacitors have been strongly required to be free from aggregation therebetween and have an excellent dispersibility as well as a high crystallinity.
For example, upon producing a laminated capacitor having a less temperature-dependency, barium titanate used as raw particles therefor must be prevented from undergoing a particle growth thereof. In particular, in order to obtain a high-capacity capacitor, the raw barium titanate particles used therefor are required to exhibit a tetragonal system.
Also, in order to allow the raw particles to exhibit an excellent dispersibility, the barium titanate particles are required to have a spherical particle shape, in particular, it is required that the barium titanate particles have a particle shape as close to a complete sphere as possible.
On the other hand, in recent years, barium titanate has been used as one of materials for forming an inner electrode layer of the laminated ceramic capacitor (Japanese Patent Application Laid-open (KOKAI) No. 2004-200450, etc.).
As the method for producing barium titanate particles, there are conventionally known a solid-phase reaction method of mixing a titanium compound with a barium compound and then calcining the resultant mixture at a temperature as high as not less than 1000° C., and a wet reaction method of reacting barium and titanium with each other in a solution thereof.
However, in the solid-phase reaction method, the resultant calcined barium titanate particles have a large average particle diameter and, therefore, must be pulverized upon use, resulting in broad particle size distribution and poor dispersibility thereof.
In general, in order to obtain barium titanate particles having a tetragonal system, it is required to heat-treat raw particles therefor at a high temperature for crystallization thereof, as described in “BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN”, Vol. 47 (5), pp. 1168-1171 (1974) such that “the temperature required for transforming barium titanate from a metastable cubic system into a stable tetragonal system is not lower than about 800° C.”.
Meanwhile, there are known the method of producing barium titanate particles by a solid-phase reaction (Japanese Patent Application Laid-Open (KOKAI) No. 2001-316114), the method of producing barium titanate particles by a wet reaction (Japanese Patent Application Laid-Open (KOKAI) Nos. 61-31345 (1986), 7-277710 (1995) and 2002-211926, and WO 00/35811) or the like.
At present, it has been strongly required to provide fine barium titanate particles satisfying the above various requirements. However, such fine barium titanate particles have not been obtained until now.
That is, in Japanese Patent Application Laid-Open (KOKAI) No. 2001-316144, there is described the method of producing barium titanate particles by heat-treating a mixture of barium carbonate and titanium oxide under a low-oxygen partial pressure. However, the resultant barium titanate particles as a high-temperature heat-treated product have a polygonal particle shape and tend to suffer from sintering therebetween, thereby failing to provide barium titanate particles having a high dispersibility.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 61-31345 (1986), there is described the method of producing barium titanate particles by a hydrothermal reaction. However, in this method, neither crystallinity nor aggregated condition of primary particles of the barium titanate particles is taken into consideration. Therefore, the resultant barium titanate particles may fail to exhibit an excellent dispersibility.
In Japanese Patent Application Laid-Open (KOKAI) No. 7-277710 (1995), there are described the barium titanate particles having a primary particle diameter and a secondary particle diameter which are substantially identical to each other. However, the resultant barium titanate particles are in the form of a cubic system and, therefore, may fail to exhibit a high crystallinity.
In addition, in WO 00/35811, there is described the method of synthesizing fine barium titanate particles by heat-treating an alkaline solution containing a brukite-type titanium oxide and barium. However, the resultant barium titanate particles are in the form of a cubic system and, therefore, may fail to exhibit a high crystallinity.
Further, in Japanese Patent Application Laid-Open (KOKAI) No. 2002-211926, there is described the method of producing tetragonal-system barium titanate particles in which after producing cubic-system barium titanate particles by a hydrothermal reaction, the thus obtained cubic-system barium titanate particles are calcined and transformed into the tetragonal-system barium titanate particles. However, in this method, it is required to conduct the calcination treatment at a high temperature for crystallization thereof, resulting in occurrence of sintering between the particles. Therefore, the obtained barium titanate particles may fail to exhibit an excellent dispersibility.
As a result of the present inventors' earnest study for solving the above problems, it has been found that spherical fine barium titanate particles having a specific average primary particle diameter, a specific ratio between the average primary particle diameter and an average secondary particle diameter, and a specific sphericity, have a small behavior particle diameter, are readily monodispersed and free from aggregation therebetween, and exhibit excellent dispersibility and dielectric characteristics. The present invention has been attained on the basis of this finding.